1. Field of the Disclosure
The present disclosure relates to an improved method of determining, while drilling in the earth with a drill bit, the positions of geologic formations in the earth. More particularly, it relates to a method for improving the quality of the processed data.
2. Description of the Related Art
Conventional reflection seismology utilizes surface sources and receivers to detect reflections from subsurface impedance contrasts. The obtained image often suffers in spatial accuracy, resolution, and coherence due to the long travel paths between source, reflector, and receiver. In particular, due to the two-way passage of seismic signals through a highly absorptive near surface weathered layer with a low, laterally varying velocity, subsurface images are poor quality. To overcome this difficulty, a technique commonly known as vertical seismic profiling (VSP) was developed to image the subsurface in the vicinity of a borehole. With VSP, a surface seismic source is used and signals are received at a single downhole receiver or an array of downhole receivers. This is repeated for different depths of the receiver (or receiver array). In offset VSP, a plurality of spaced apart sources are sequentially activated, enabling imaging of a larger range of distances than would be possible with a single source.
VSP measurements made during drilling operations are referred to as Seismic-while-drilling. The signals generated by seismic sources are reproducible and could be stacked. The other kind of data recorded by the seismic sensors is noise. We distinguish between background noise (drilling noise, circulation noise, rig noise, cultural noise, environmental noise) and spiky noise (e.g. due to hitting the drill string while connection; micro earthquakes close to the borehole). It is helpful to restrict seismic recording of data to low-noise periods. It is also helpful to stack the data. Because the noise is random, stacking the data increases the signal to noise ratio.
U.S. Pat. No. 7,299,884 to Mathiszik, having the same assignee as the present disclosure and the contents of which are incorporated herein by reference, discloses a method of making seismic measurements during drilling of a borehole by identifying suitable intervals when data quality are likely to be good, and making seismic measurements during those time intervals. Measurements may be made continuously with a seismic while drilling (SWD) system and the measured data may be stored in a working memory of a downhole processor along with quality control (QC) measurements. The QC data are analyzed and, based on the analysis, portions of the data in working memory may be recorded in permanent memory for retrieval. Alternatively, QC measurements may be made substantially continuously, and predictions may be made when data quality for SWD measurements are likely to be good. Recording of SWD data are then started based on the prediction.
The Seismic-while-drilling measurements include the VSP-while drilling (VSP-WD) method and the checkshot-WD (CS-WD) method. These methods permit updating of the geological model. In this way one can reduce the drilling risk and/or update the optimal well path. The importance of real-time processing is evident, but a full real-time processing isn't possible yet. One limitation of Seismic-while-drilling measurements is the small bandwidth of the uplinks and downlinks. The communication is done via mud telemetry, which is possible only while circulating. Hence when tripping in or tripping out, measurements are done without circulating between shooting windows, and it is not possible to send uplink signals and downlink signals. Even when mud telemetry is possible, the bandwidth available for uplink signals and downlink signals is very small.
Due to the small uplink bandwidth, the downhole tool must automatically detect and process the shooting sequence(s) downhole. Only the final results (e.g., the first-break time) are sent to the surface.
Because Seismic-while-drilling measurements need a quiet environment, they are done during natural drilling breaks (e.g., while connecting drill pipe segments). The measurement window might be set by the downhole tool itself (e.g. U.S. Pat. No. 7,299,884 to Mathiszik, having the same assignee as the present disclosure and the contents of which are incorporated herein by reference), or by the operator by sending a downlink. In both cases the measurement window is larger than the window of the shooting sequence, and the downhole tool must detect the exact shooting sequence automatically.
A robust detection of shooting sequences is very important if more than one sequence is performed during a measurement window. Usually, the length of a measurement window depends on the drilling mode (drilling or tripping). While drilling, the measurement window is short and often related to the connection time. Usually only one shooting sequence is performed during a measurement window, but several shooting sequences are also possible. While tripping, the measurement window usually includes several shooting sequences. Very often, one measurement window covers the whole tripping time. During the trip-in mode tens of shooting equences are performed. When the trip-in mode is finished, it is important that the results of several shooting sequences should be sent to the surface as soon as possible. A prerequisite for processing of a shooting sequence is the detection of the shooting windows. Of course, the exact shooting window(s) can be communicated with a downlink after the measurement(s). However, this procedure introduces a delay in the “real-time” processing, and sending an additional downlink is also very expensive.
There is a need for a method of automatically detecting the shot breaks downhole substantially in real-time. The present disclosure addresses this need.